1. Field of the Invention
The present invention relates to an image forming apparatus, and a method for controlling a driving condition of a belt driven in a condition having a periodic disturbance.
2. Description of the Related Art
An image forming apparatus such as a copier and a printer is known to have a structure using an intermediate transfer belt configured to superimpose toner images formed on photoconductive drums of respective colors and to transfer and form the superimposed image to a recording member such as a sheet. Because the intermediate transfer belt is wrapped around a drive roller, a tension roller and others, the belt is apt to meander or to lean to one-side in a belt widthwise direction during its travel due to such disturbances caused by imprecision of the rollers and of parallelism of the belt and a distribution of tension of the belt itself, and a disturbance caused when the recording member rushes into the belt. The meandering of the belt and the leaning of the belt to one-side in the belt widthwise direction will be referred simply as a “shift of widthwise position” or a “shift” hereinafter.
Because this shift causes misregistration of the respective color images in composing the respective color images, the image forming apparatus is arranged to correct the shift of the belt by executing steering control. The steering control is an operation for correcting the shift of the belt by detecting a widthwise position when the belt is shifted or shift speed of the intermediate transfer belt by sensors and by carrying out feedback control of slanting a specific roller (referred to as a “steering roller” hereinafter) based on detected values.
It is also known that speed of shift of the belt caused by the slant of the roller of the steering method is proportional to moving speed of the belt in a rotational direction (referred to as “belt moving speed” hereinafter). This indicates that behaviors of the belt in the widthwise and rotational directions are linked with each other, so that it is necessary to take this linkage into account in order to control the shift (widthwise position) of the belt in high precision.
Taking such linkage into account, a configuration that translates feedback gains of the belt shift control into a variable gain control system regarding the belt moving speed is being proposed. According to the configuration, an adjustment of a feedback control system of the belt shift control is made first with a normal belt moving speed called a belt reference speed. However, if the belt moving speed varies after that and differs from the belt reference speed, the shift feedback control system is destabilized because an amount of shift per unit time varies. If the belt moving speed increases as a result, a loop gain of the shift feedback control system becomes too high and a response of the shift starts to oscillate. Then, Japanese Patent Application Laid-open No. 2008-111928 stabilizes a closed loop by multiplying the shift feedback control system by a value obtained by dividing the belt reference speed by the belt moving speed. This method will be referred to as a “variable gain method” hereinafter.
Meanwhile, it is effective to feedforward control the steering roller on the timing when a recording member rushes into the belt to suppress a shift caused by a sudden disturbance (other disturbance) such as the inrush of the recording member. To that end, Japanese Patent Application Laid-open No. 2005-107118 proposes a configuration that estimates the timing when the recording member rushes into the intermediate transfer belt by using sensors for detecting the recording member and implements the feedforward control on the belt moving speed. This configuration prevents the belt moving speed from dropping when the recording member rushes into the belt by executing such feedforward control.
The variable gain method described above in Japanese Patent Application Laid-open No. 2008-111928 is effective under a condition in which the belt moving speed fluctuates in ramp due to a periodic disturbance caused by decentration or the like of the suspension roller. However, if the belt moving speed drops oscillatively and suddenly due to the other disturbance such as the inrush of the recording member, there is a possibility that a gain of the feedback control system becomes high, considerably varying a steering amount.
Still further, the method for controlling the belt in terms of its traveling direction described in Japanese Patent Application Laid-open No. 2005-107118 will do just by generating a sole feedforward input corresponding only to a condition if the condition is that the same type of recording member rushes into the belt at constant speed. However, in the control of the shift of the belt, although a large deviation of the widthwise position is generated if a steering amount is large when the disturbance occurs due to the inrush of the recording member, almost no deviation of the widthwise position is generated when the steering amount is small when the disturbance occurs. Thus, this shift feedforward control of this method has a problem that a large number of feedforward inputs have to be generated and stored in advance even under the condition that the same type of recording members rush into the belt at a constant speed.